This invention relates generally to the measurement of the area of apertures in an opaque medium and particularly to the measurement of the area and dimensions of the slits in a kinescope shadow mask.
Kinescopes for color television receivers typically contain a shadow mask which serves as a color-selection electrode. The shadow mask is made of thin metal and contains a plurality of apertures, typically in the form of substantially rectangular slits. The apertures are arranged in a systematic pattern and are useful in assuring that the three electron beams of the kinescope impact phosphor which emit the proper colors of light. The apertures are placed into the shadow mask material by acid etching. The shadow mask material is coated on both sides with a photoresist material. The aperture pattern is then photographically produced on both sides of the material by exposure to light. The unexposed photoresist material is then washed away and to leave bare metal which is subsequently etched away in an acid tank to form the apertures.
A knowledge of the area and dimensions of the etched apertures is desirable for several reasons. For example, the desired optimum area of the apertures is known and, accordingly, a substantial deviation from the desired area can be indicative of the need to correct one or more parameters of the shadow mask fabrication processes. Additionally, shadow masks which have apertures within a permissible range of tolerances are grouped into categories in accordance with the area of the apertures for automatic processing purposes. Prior art efforts at measuring aperture areas have measured the ability of the shadow mask to pass light. Thus a known intensity of light is used to illuminate the shadow mask and the intensity of light passing through the apertures is taken as a measure of the area of the apertures. This technique is unacceptable because acceptable readings can be obtained from unacceptable aperture. For example, the apertures can be too wide and too short for acceptable tube operation but still have an area equal to that of dimensionally acceptable apertures. For these reasons, there is a need for a system for accurately measuring the average area and dimensions of the apertures etched into shadow masks. The instant invention fulfills this need.